The present invention relates in general to the field of methods for controlling a solenoid valve and regards in particular a method for controlling a solenoid valve that can be used in a system for variable actuation of the valves of an internal-combustion engine according to a multi-lift mode.
A system for variable actuation of the valves of an internal-combustion engine according to a multi-lift mode has formed the subject of European patent application EP 1 726 790 A1 filed in the name of the present applicant. The system disclosed in this document relates to an internal-combustion engine of the type comprising at least one cylinder and at least one intake valve and one exhaust valve associated to the cylinder, and actuated so as to control the flow of gas through the respective intake and exhaust pipes, in which at least one engine valve is controlled by electronically controlled variable-actuation means, designed to impart on the valve different opening and closing times and different lift profiles, as the conditions of operation of the engine vary.
In recent years an ever-increasing number of studies and experiments has been developed in the field of engines of the type specified above. The present applicant is the owner of a large number of patents and patent applications regarding a system for variable actuation of the valves, in which each variable-actuation valve is controlled by the respective cam by means of a tappet and corresponding hydraulic means, which include a pressurized-fluid chamber, for hydraulic transmission of the movement of the tappet to the engine valve. The aforesaid pressurized-fluid chamber can be connected by means of an electronically controlled solenoid valve to an exhaust channel, so that the transmission between the cam and the engine valve is interrupted and the engine valve closes as a result of the respective biassing spring means. The solenoid valves that control communication of the pressurized-fluid chambers associated to the various engine valves with the aforesaid exhaust channel are electronically controlled according to different possible strategies, as the conditions of operation of the engine vary, to obtain advantages in terms of performance and/or efficiency of operation of the engine and/or reduction of the consumption of the fuel and/or reduction of the noxious exhaust emissions.
The system forming the subject of EP 1 726. 790 A1 envisages control of the intake valves of the engine, preferably with the aid of an electro-hydraulic system of the type specified above, to impart on each intake valve, in predetermined conditions of operation of the engine, a plurality of successive cycles of movements of opening and closing within what would be the single conventional cycle of opening and closing of the engine valve.
In the specific case in which this operating mode is implemented with the aid of an electro-hydraulic system of the type described above, there arises a problem due to the fact that each of the solenoid valves used in this system is of the normally open type, in which a valve element can be driven to the closed condition by means of a pusher element which is rigidly connected to the movable core associated to the solenoid of the solenoid valve, this pusher element being separate, instead, from the body of the valve element. As will emerge more clearly from the detailed description of the solenoid valve that will be provided hereinafter, this results in that, when the solenoid valve is de-energized, following upon a closing thereof, the valve element returns to its starting end position, whilst the pusher element continues its return stroke, detaching from the body of the valve element and then takes time to return again into contact with the valve element, as a result of the action of a respective return spring. This structure of the solenoid valve thus sets a minimum limit to the time that must elapse between two successive energizations of the solenoid valve in so far as, for the purposes of proper operation and long duration of the solenoid valve, it is necessary for each new energization of the solenoid valve to occur after the pusher element has returned to its resting position, in contact with the valve element, following upon the preceding de-energization. For example, in the specific case of a solenoid valve used by the present applicant, the aforesaid minimum time that must elapse between two successive energizations of the solenoid valve is 2.5 ms. This represents a limit with respect to the possibility of an optimal exploitation of the system for variable actuation of the engine valves in so far as in the multi-lift actuation mode there may occur, in certain conditions of operation of the engine, the need to control a second subcycle of opening of the engine valve after the end of the preceding subcycle without waiting for the aforesaid minimum waiting time to elapse.